The present invention generally relates to power converters and, more specifically, to high-power, low-to-high voltage DC-to-AC and DC-to-DC voltage boosting power converters.
There are many applications, particularly in avionics, that require high-voltage (in the area of 10 kV), high-power output (in the area of 1.5 kW) power supplies. Often due to size, weight, or other limiting factors, the high-voltage power output must be generated from an existing, relatively lower voltage power supply. In such cases, a step-up or low-to-high-voltage boosting converter is typically used. Preferably, this boost converter should be capable of a wide range of output voltages so as to be adaptable to a wide variety of applications. Further, the design of the converter must necessarily be such that it does not interfere in the operation of the device being powered.
One class of commonly used power converters includes the numerous circuit variants of the basic inductive switched mode DC-to-DC voltage boosting converter. The basic switched mode converter is generally characterized as having a voltage gain that is directly related to the duty cycle of an applied converter control signal. This converter also characteristically has an output voltage ripple frequency that is approximately equal to the converter switch drive signal frequency. Voltage gain is here defined as the ratio of the converter output voltage to the converter input voltage. Duty cycle is here defined as the conduction period of the converter switch, responsive to the converter switch drive signal, to the total switch cycle period. As is well-known in the art, the voltage gain of the converter is given by the equation: EQU V.sub.g =1/(1-D)
where V.sub.g is voltage gain and D is duty cycle. Thus, based on this equation, an arbitrarily high-voltage gain can theoretically be obtained by the use of a converter switch drive signal having a duty cycle approaching unity. Also theoretically, any desirably high frequency of ripple in the output voltage potential, as required for non-interference with the device being powered, might be achieved by using a correspondingly high converter switch drive signal frequency.
In actual practice, however, there are a number of limitations on the voltage gain and ripple frequency, as well as the maximum output voltage and current, that can be realized when using the basic switched mode converter. In particular, both the maximum duty cycle and frequency of the converter switch drive signal are effectively limited by converter component and circuit non-idealities. That is, the power conversion efficiency and voltage gain of the converter begin to deteriorate rapidly whenever either an excessively high converter switch drive signal frequency or a duty cycle closely approaching unity, or both, are used.
The maximum output voltage and current are also limited by the converter components used. A typical switched mode converter utilizes an inductor to boost the converter input voltage. A switch, typically a transistor, is employed to charge the inductor by periodically shorting the converter input voltage through the inductor to ground or another reference voltage potential. In turn, when the switch is not conducting, the inductor impresses a boosted voltage potential across the switch. Thus, it is the collector to emitter, or equivalent, breakdown voltage and peak collector, or equivalent, current capacity that directly limits the maximum steady state voltage and current output of the converter. Consequently, the power transfer capability of the basic switched mode converter is also limited.
Another limitation associated with the use of the basic switched mode converter is that it does not provide a power isolated output. Power isolation is typically required between devices powered by the high-voltage power supply and those powered by the low-voltage power supply, from which the high-voltage supply is derived. Power isolation decreases the possibility of undesirable interference between the devices being powered and correspondingly increases their functional reliability.
Numerous circuit variants of the basic switched mode converter have been developed to avoid the above limitations. These variants typically employ an AC coupling transformer to provide power output isolation. An additional benefit of using an isolation transformer is that it can also be used to increase the voltage gain of the converter. By increasing the turns ratio of the secondary winding to the primary winding, the voltage potential across the secondary is proportionally increased with respect to the primary. Further, through the use of multiple secondary windings, the transformer can provide several isolated power outputs.
More elaborate variants have been designed specifically to provide high-voltage power outputs having substantially reduced output voltage ripple. A representative sampling of these circuit variants is shown in U.S. Pat. Nos. 4,251,087 and 4,184,197. Typically, these converter variants use either inductive filtering or inductively coupled feedback, or both, to improve the stability of the output voltage potential. This effectively reduces the possible interference of the switched mode converter with the operation of the device being powered and, therefore, avoids the need for excessively high converter switch drive signal frequencies.
These improved variants of the basic switched mode converter, however, are not without their disadvantages. For example, the addition of the transformer requires the converter circuit design to be modified so as to prevent DC saturation of the transformer core. DC saturation, as is well-known, would effectively inhibit the operation of the converter. This naturally results in an increase in converter design complexity, size, and weight and a corresponding decrease in converter power conversion efficiency.
Use of the transformer turns ratio to increase voltage gain also has the disadvantage of decreasing converter power conversion efficiency. The parasitic capacitance inherently present in the secondary winding is multiplied by the turns ratio squared and effectively reflected into the primary winding. This multiplied capacitance thereby acts as a substantial capacitive power load on the low-voltage power supply. Since this power is consumed by the converter, the overall power conversion efficiency is decreased.